Development of a calculation program for escort forces of stern drive tug boats

The escort class notation of several classification societies requires the test of the dynamic towing forces at escort speeds of 8 or 10 knots. These tests should be performed with the completed tug according to a special procedure. Accurate prediction methods are available mostly for tractor type tug boats. As more and more stern drive tugs (ASD-tugs) are built, aiming also for an escort class notation, SCHOTTEL and the Dutch Model Test Establishment MARIN have developed a computer prediction program for the escort forces of these tugs. The program is based on executed escort tests and systematic model tests with different configurations of the hull, skeg, arrangement of thrusters and towing positions. The program allows optimising the above configurations and it helps to predict the escort rating number of ASD-tugs in a very early stage of the design.